This invention relates to a power supply system including an operating circuit which is for operating a CRT display unit, a printer unit, and the like and which is turned on and off in response to supply and interruption of electric power and, in particular, to a power control circuit for controlling the electric power supplied to the operating circuit.
Generally, a power supply system of the type described is used to operate peripheral units of an information processing device such as a computer. Among such peripheral units, a CRT display unit, a printer unit, or the like is put into operation in response to an input signal, such as a synchronization signal, a video signal, and a control signal, which is supplied from an external apparatus. When a peripheral unit of the above-mentioned type is used, it is preferable in view of a reduction of power consumption to turn off a power source circuit for the peripheral unit in absence of the input signal and to turn on the power source circuit only in the presence of the input signal.
To this end, the power supply system generally includes not only an input signal source and an operating circuit such as the power source circuit for the peripheral unit but also a power control circuit for turning on and off the operating circuit in response to presence and absence of the input signal.
Such a conventional power control circuit for carrying out the above-mentioned operation comprises a detecting circuit for detecting non-reception of the input signal, and a switching circuit for turning the operating circuit into a stop mode, namely, an off state when the detecting circuit detects non-reception of the input signal. When the input signal is received again, the operating circuit must be recovered. Therefore, the power control circuit is required to additionally comprise a reception detecting circuit for detecting reception of the input signal and an auxiliary power source circuit for supplying electric power to the reception detecting circuit.
In case where the above-mentioned power control circuit is used, the reception detecting circuit must be kept in an active mode by the auxiliary power source circuit even if the input signal is not received. This results in a complicated circuit structure and an increase of power consumption.